My 2006 chevy trailblazer is giving me problems.. I turn my truck on and it idles at a little over the 40 mark on my oil pressure gauge. Once i put truck in gear, the oil pressure drops slowly to 0.. Then the engine start knocking.. A couple people say it can be the oil pump, or a clog in oil system.. What is it? Do I need an Oil flush, or should I change my Oil pump? Or is there any other Suggestions?